The present invention relates to an apparatus for measuring the flow of a fluid including freely displaceable electric charges flowing through a substantially horizontally arranged measuring tube. The apparatus includes magnetic means for producing at least two states of a magnetic field in which the magnetic field extends through the measuring tube substantially mirror symmetrically relative to a vertical plane intersecting the axis of the measuring tube. The magnetic field in a first state extends in a horizontal plane intersecting the measuring tube. The magnetic field in the first state extends in substantially the same direction on both sides of the horizontal plane and in a second state extends in substantially opposite directions on both sides of the horizontal plane. At least one point of the second state of the magnetic field has a zero value and is located in the vertical plane and the horizontal plane. A single pair of electrodes are arranged at the measuring tube on both sides of the vertical plane to sense displacement of charges in the fluid. A circuit means corrects the voltage between the electrodes measured in the first state of the magnetic field, which voltage is usually only approximately proportional to the flow of the fluid, by using the voltage between the electrodes measured in the second state of the magnetic field to obtain an output signal substantially proportional to the flow of the fluid.
In a known apparatus of this type, the electrodes are located in a horizontal plane identical with the mentioned horizontal plane of both magnetic field states.
In the known apparatus it turned out that there is no definite correlation between the first-mentioned voltage and the flow of the fluid, but that this correlation rather is dependent on the flowing profile of the fluid.
One and the same value of the first voltage, therefore, requires different corrections dependent on the flowing profile. Such corrections cannot be made by the circuit means of the known apparatus because this apparatus is unable to detect different flowing profiles.